1. Field of the Invention
The present invention relates to a fuel injection valve which injects a fuel from a nozzle hole, and more particularly to a fuel injection valve which generates a cavitation bubble in fuel flowing inside an injection valve.
2. Description of the Related Art
Fuel injection valves which generate cavitation bubbles in fuel flowing inside an injection valve in order to enhance atomization in a fuel spray have been proposed. Japanese Patent Laid-Open Publication No. 2003-83205, Japanese Patent Laid-Open Publication No. 2004-19481, and N. Tamaki et al., “Atomization Enhancement of the Spray and Improvement of the Spray Characteristics by Cavitation and Pin Inserted in the Nozzle Hole”, ICLASS, 2003 describe related art of such a structure. The fuel injection valve of Japanese Patent Laid-Open Publication No. 2003-83205 has a cavitation generator which generates cavitation bubbles in the fuel and a cavitation eliminator which eliminates the cavitation bubbles generated by the cavitation generator. In the fuel injection valve of this reference, disturbance is caused in the fuel flow within the nozzle hole by an impact pressure which is generated during disappearance of the cavitation bubbles to enhance atomization of the fuel spray.
In a fuel injection valve according to Japanese Patent Laid-Open Publication No. 2004-19481, the nozzle hole is separated into a first nozzle hole portion on the upstream side and a second nozzle hole portion on the downstream side. By setting the cross sectional area of the second nozzle hole portion to be larger than the cross sectional area of the first nozzle hole portion, a storage portion which stores the fuel is formed between the inner wall of the second nozzle hole portion and the fuel jet flowing from the first nozzle hole portion. Cavitation bubbles are generated within a shearing layer which is created by a velocity difference between the fuel stored in the storage portion and the fuel jet flowing from the first nozzle hole portion. In this manner, cavitation bubbles are formed near an outer peripheral surface of the fuel jet and the energy when the cavitation bubbles collapse is used for atomization of the fuel spray.
In the fuel injection valve of N. Tamaki et al., the nozzle hole is configured so that a gap portion is provided between an upstream nozzle hole and a downstream nozzle hole. Cavitation bubbles generated by the upstream nozzle hole collapse in the gap portion due to attenuation of the fuel flow and recovery of pressure. In addition, because a projecting pin is provided inside the nozzle hole, the cavitation bubbles also collapse in the downstream nozzle hole. A disturbance is caused in the fuel flow within the nozzle hole by the collapse of the cavitation bubbles so that the atomization in the fuel spray is enhanced.
In order to effectively achieve the atomization enhancement effect of the fuel spray by cavitation collapse over the entire region of the fuel jet after injection, it is desirable to inject fuel in which the cavitation bubbles are uniformly mixed (or mixed in an approximate uniform manner) from the nozzle hole.
In the fuel injection valves of Japanese Patent Laid-Open Publication No. 2003-83205 and N. Tamaki et al., because the cavitation bubbles disappear within the injection valve, only the fuel in liquid from is present in the fuel jet downstream of the exit of the nozzle hole. Therefore, the atomization enhancement effect of the fuel spray by the cavitation collapse cannot be obtained in the fuel jet after injection.
In the fuel injection valve of Japanese Patent Laid-Open Publication No. 2004-19481, although fuel in which the cavitation is mixed can be injected from the nozzle hole, the formation region of the cavitation bubbles is limited to a region near an outer peripheral surface of the fuel jet and a core of liquid phase remains around the center of the fuel jet. Because of this, the atomization enhancement effect of the fuel spray by the cavitation collapse cannot be obtained in a wide area in the fuel jet after injection.